Plasma etchers are frequently used in semiconductor processing when anisotropic etching is needed to produce a relatively straight vertical edge. For instance, when etching the polysilicon gate of a MOS transistor, a sloped polysilicon edge can adversely affect the operation of the transistor. This is referred to as undercutting. Undercutting is frequently encountered when etching is performed using a liquid etchant. Plasma etching, which uses gaseous ions accelerated by an electric field, tends to etch only horizontally exposed surfaces and therefore avoids undercutting.
An important aspect of all etching processes is stopping the etching process after the layer being etched has been removed but before the next layer down is destroyed. This is often called "endpoint" detection--for detecting the completion of etching of a particular layer.
An example of a situation in which precise endpoint detection is important in fabricating MOS transistors is the processing step in which a thin "spacer" is formed adjacent to an MOS transistor gate. Referring to FIGS. 1A-1D, in FIG. 1A a silicon oxide film is deposited over a polysilicon gate that was defined by a prior etch step. FIG. 1B shows the results of a properly terminated plasma etch of the silicon oxide layer, producing silicon oxide spacers on all sides of the polysilicon gate. FIG. 1C shows that the channel width L1 of the transistor is governed by the width of the silicon oxide spacers. FIG. 1D shows that if the plasma etching of the silicon oxide layer is allowed to continue past its ideal endpoint, the silicon oxide spacers will be thinner than is ideal and resulting transistor channel width L2 will be shorter than the channel width L1 that was supposed to be produced.
Referring to FIG. 2, there is shown a triode etcher system 100, such as a GCA Waferetch 616 triode etcher, having state of the art "endpoint" detection equipment. The system 100 has an etching chamber 102 with upper and lower cathodes 104 and 106, respectively, and a screen anode 108. The screen anode 108 is located between the two cathodes and is grounded. A semiconductor wafer 110 is placed in the chamber on the lower cathode 106. In this example the wafer 110 has a silicon substrate 112 that supports a dielectric layer 114. The dielectric layer 114 has been masked with resist 118 in order to define regions of dielectric layer 114 to be etched. In other situations, such as that shown in FIGS. 1A and 1B, no mask is used, and in still other situations etch masks are formed with materials other than resist. The interior of the etching chamber is filled with a gaseous mixture that becomes a plasma 120 when power is applied to the chamber.
The plasma etching system shown in FIG. 2 has a 13.56 megahertz RF power supply 130 with a characteristic impedance of 50 ohms. The chamber 102, however, has a characteristic impedance of around 1000 ohms at this frequency. Without the use of a compensating circuit, this impedance mismatch would cause most of the power output by the power supply to be reflected off the load (i.e., the chamber) and back to the source, which could damage the power supply 130. To overcome this problem, most or all etching systems use a compensating circuit 132, sometimes called an impedance transformation circuit, which matches the amplifier to the plasma. In a triode etcher such as the one shown in FIG. 2, this circuit typically includes an inductor coil L1 and three tunable capacitors C1, C2 and C3. A controller 134 (typically a programmed, digital microcontroller) automatically monitors the reflected power and adjusts the three capacitors until the reflected power is less than a specified threshold value.
In general, the plasma 120 etches the top layer of the wafer 110 only when the power supply 130 is activated and when the power reflected by the plasma chamber is relatively low. Activating the power supply 130 "strikes" the plasma, and activates the etching process. While etching any particular layer, light is generated at wavelengths corresponding to the reactants (components of the gaseous mixture) used and to the products of the reaction between the components of the plasma and the layer being etched.
In many plasma etching systems the endpoint of the etching process is detected using a light or optical sensor 140 and a digital computer in controller 134. Typically, the optical sensor 140 is set up, using narrow bandpass filters, to monitor the intensity of light at a single characteristic wavelength associated with the reaction products produced by etching the layer or to monitor the intensity of light at a wavelength associated with the gaseous reactants in the plasma used to etch the layer. When the measured intensity falls below a specified threshold or rises above a specified threshold, depending on the wavelength being monitored, the controller 134 generates an endpoint signal that turns off the power supply 130 and thereby stops the etching process.
In conventional etching systems such as that depicted in FIG. 2 the endpoint signal may be generated too early or too late for stopping the etch process at the optimal point. In some instances the delay between completion of the dielectric etch and the subsequent disengagement of the power supply 130 has allowed the plasma to significantly damage the exposed surfaces of the substrate 112. In other circumstances, particularly when etching transparent or translucent dielectric films using a plasma reactor whose optical path for the endpoint detection system is not parallel to the wafer surface, a premature endpoint signal is generated. It is this latter circumstance that is addressed by the present invention.
Many plasma reactors utilize optical endpoint detection systems whose optical path is parallel to the wafer surface, and those systems generally do not experience false, premature endpoint detection signals. However, providing an optical endpoint detection system whose optical path is parallel to the wafer surface generally requires an increased reactor volume.
It is therefore an object of the present invention to provide a reliable endpoint detection method for terminating the etching of dielectric films in plasma reactors that utilize optical endpoint detection apparatus which is not restricted by requiring an optical path parallel to the wafer surface.